A variety of disorders and therapies involve a deficiency of immune cells. For example, HIV infection results in a loss of CD4+ T-cells, while therapies such as chemotherapy and radiation therapy generally result in a loss of a wide variety of blood cells. Attempts have been made to provide specific protein drugs that can replenish specific types of immune cells that are lost as a result of a disease or therapy. For example, in cancer chemotherapy, erythropoietin is used to replenish red blood cells, granulocyte colony-stimulating factor (G-CSF) is used to replenish neutrophils, and granulocyte macrophage colony stimulating factor (GM-CSF) is used to replenish granulocytes and macrophages. These protein drugs, although beneficial, have relatively short serum half-lives such that immune cell replenishment is often insufficient. Moreover, no specific treatment is currently in clinical use to specifically stimulate T or B-cell development, even though loss of these cells as a result of disease or after certain myeloablative treatments is know to be particularly deleterious to a patient's health. Thus, there exists a need in the art to develop immune system stimulators and restoratives, particularly of lymphocytes, that have extended serum half-lives.